1. Gold skarn Deposits
- Geology and Exploration Criteria -

2. Object To understand and classify Skarn deposits
To understand geology,mineralogy, characteristics, and distributions from main Skarn gold deposits
Suggest relevant exploration criteria for economic skarn gold deposits

3. Classification of gold deposits
Intrusion-related gold deposits
Carlin-type gold deposits in Nevada, USA
Epithermal gold deposits in volcanic terranes

4. “Skarn”
Originally a term applied to coarse-grained calc-silicate gangue associated with the iron ore deposits of Sweden
It include a variety of calc-silicate rocks rich in calcium, iron, magnesium, aluminium, manganese that formed by replacement of originally carbonate-rich rocks.
Skarn deposits result from the hydrothermal interaction of hot silicate magmas and cooler sedimentary rocks

5. Fig 1. Stages in the development of skarn deposits

6. The type of skarn deposits (Enaudi et al., 1981)
Gold type
Iron type
Copper type
: nonpophyritic, small-size, high- gold grade in high sulfide content, intense retrograde alteration
Porphyry copper type
: Large-scale and low-gold grade
Lead-zinc type
: very little gold contain

7. Fig.2 Distribution of Au, Ag, and Cu in skarn, pluton, and protolith of a large copper skarn system, Whitehorse copper belt, Canada (from Meinert, 1986)

8. Skarn gold deposits
Related to copper-mineralized porphyritic intrusions
Usually clastic or volcanoclastic component
Age : Cambrian or older ~ Miocene
Arsenopyrite and pyrrhotite are abundant
Typically contain As, Bi, Te

9. Fig. 3. Electron microprobe analysis (mole%) of garnet and pyroxene from Fortitude, Nevada ; Hedley, British Columbia; Mccoy, Nevada: west central Montana gold skarn.

10. Table 1. Gold Distribution in Gold Skarn Deposits
Localty
Size(t)
Au(g/t)
Ag(g/t)
Cu(%)
Au/Cu
Au(kg)
Ag(kg)
Cu(tons)
Bau
2,400,000
7.2
0.1
<0.1
17,280
240
Beal
9,200,000
1.5
1.3
0.0
37.7
13,883
11,960
3,680
Brown’sCreek(u.g)
450,000
5.2
9.0
0.4
11.8
2,340
4,050
1,980
Brown’sCreek(0.p)
1,382,380
4.5
10.2
6,221
12,441
6,082
Cable
1,000,000
6.0
5.0
3.0
2.0
6,000
5,000
30,000
Fortitude
10,300,000
6.9
24.7
86.6
71,379
254,410
8,240
Golden Curry
930,000
8.5
4.2
0.3
25.8
7,905
3,906
3,069
Hedley(o.p)
8,900,000
4.6
1.4
45.6
40,584
12,460
8,900
Hedle yu.g)
3,604,849
13.5
224.9
48,642
10,815
2,163
La Luz
16,000,000
4.1
1.2
9.3
65,600
19,200
70,400
Marn
100,000
2.8
4.7
140
280
Maura Sipongi
113,000
5.6
2.5
0.2
28.0
633
283
226
McCoy Creek
14,500,000
15.0
21,750
1,450
14,500
Minnie-Tomboy
3,900,000
10,920
35,100
11,700
Red Dome
15,000,000
2.6
0.5
5.7
39,000
69,000
Silver Star
small
50.0
4.0
12.5
Southern Cross
400,000
13.0
16.0
147.7
5,200
6,400
352
Suian
530,000
4.9
6,890
2,597
Thanksgiving
1,700,000
6.4
40.6
17.8
10,897
68,935
6,120
Tillicum
126,000
20.6
2,596
Tul Mi Chung
12.0
4,800

11. Table 2. Geologic Characteristic of Gold skarn Deposits
Contained
Au, tones
Age, Ma
Ore-related
intrusion
Host rocks
Metal
Association
Skarn
Mineralogy
Fortitude,
Nevada,
USA 96
37.2
Porphyritic granodiorite stock and dykes
Carboniferous-Permian limestone
Au-Ag-Cu-Zn-Pb-As-(Bi-Te)
Gar,pyx,ep,act,chl
McCoy, 30
39.7
Triassic limestone
Au-Ag-(Cu)
Gar,pyx,ep,wo,scp,adularia
Hedley,
British Columbia,
Canada
83.5
Early
Jurassic
Diorite and quartz diorite porphyry sills and dykes
Triassic calcareous and tuffaceous siltstone
Au-Cu-As-Zn-(Co-Ni-Bi-Mo-Te)
Ad,di,scp,axinite,qtz,ep,tr,cz,chl,kspar
Red Dome,
Queensland,
Australia 39
Caboni-ferous
Rhyolite porphyry dyke
Silurian limestone
Au-Cu-As-Zn-(W-Sn-Bi-Te)
Wo,gar,pyx,act,ep
Thanksgiving,
Philippines 13
5.5
Diorite porphyry stock and dykes
Miocene limestone
Au-Ag-Zn-(Pb-Cu-As-Te)
Gar,act-tr,ep,cz,id,chl
Suan,
North Korea
>100
Grinite pluton
Late Protetozoic-Cambrian limestone and dolomite
Cu-Au-(Zn-Pb-As-Bi)
Navachab,
Namibia 22
Cambro-
Ordovician
Leucognite dykes
Late Proterozoic dolomitic mable
Au-(Cu-Pb-Zn-W-Bi-Te-As-Mo)

12. Summary and Exploration Criteria
Skarn mineralogy (especially, garnet and pyroxene) is the key to explore skarn deposits.
The abundance of pyroxene relative to garnet and the high ferrous/ferric ratios reflect the generally reducing environment of gold skarns.
Reducing conditions and subsequent oxidation is important in skarn gold deposition

13. Most gold skarns are associated with relatively mafic reduced plutons and contain anomalous of As, Bi, and Te.
Explorationists should noted that the more proximal gold-poor, garnet-rich part of a skarn deposit is likely to be more resistant to erosion and thus more likely to crop out and be sampled than the more distal gold and pyroxene-rich skarn.
Economic gold skarns should be sought in the distal part of under reducing conditions.